1. Field of the Invention
This invention relates to a focusing device which focuses an object image on an imaging plane by controlling a photo-taking lens state and, more particularly, to a device for controlling the length of operating time of each of focusing operation processes.
2. Description of the Prior Art
Many focusing devices are known which are arranged to perform a focusing operation through the processes of receiving by light receiving means a reflection light from an object to be photographed, determining an extent of deviation of a photo-taking optical system from an in-focus position thereof on the basis of a signal produced from the light receiving means, and shifting the position of the optical system toward the in-focus position according to the result of the determining process. In the conventional focusing devices of the above-stated kind, however, the length of time (or an operating time) for each of these focusing processes has been left uncontrolled.
Comparing, for example, a case where a detected position of a lens focused on an object greatly deviates from an in-focus position with another case where a detected focused position of the lens is relatively close to the in-focus position, a longer period of time is necessary for shifting the lens position to the in-focus position in the former case than in the latter. In the device arranged to perform a focusing operation by repeating a series of processes of receiving a light--detecting a focused state--computing an extent to which a photo-taking lens is to be shifted--shifting the lens--receiving a light again . . . , a certain length of time is required before an in-focus state is obtained by shifting the photo-taking lens in the event of deviation to a large extent from the in-focus state. In that event, in-focus state cannot be obtained by carrying out a second performance of the focusing operation immediately after the first performance. It is then necessary to wait for the lapse of a certain length of time before the start of the next performance. Conversely, in the case of a focused state close to an in-focus state, the in-focus state is obtainable by a slight shift of the photo-taking lens. In the latter case, therefore, distance measuring accuracy can be increased by starting the next performance of the focusing operation within a shorter period of time to detect a new focused state and by shifting the lens according to the newly detected focused state.
Focusing devices are also known of the kind having light emitting means and arranged to enhance focused state detecting accuracy by allowing the light emitting means to intermittently emit a light. In the case of the focusing device of this kind having light emitting means, if the light emitting means is arranged to perform light emitting and no light emitting actions with the light emission arranged to be performed in a fixed cycle, the light emission might be wastefully performed in some cases depending on the focused state. In other words, in the event of great deviation of a detected focused state from an in-focus state requiring a long period of time for shifting the photo-taking lens to an in-focus position as mentioned above, the light emission from the light emitting means during the shift of the lens is meaningless. Further, in the event of a detected focused state close to an in-focus state, the distance measuring accuracy can be increased by causing the light emitting means to emit light at an increased rate of recurrence.